1. Field of the Invention
This invention relates to a radiation image read-out and reproducing method wherein a stimulable phosphor sheet having a radiation image stored thereon is exposed to stimulating rays which cause the stimulable phosphor sheet to emit light in proportion to the amount of energy stored thereon during exposure to radiation, the emitted light is photoelectrically detected and an electric image signal is thereby obtained, which electric image signal represents the radiation image and is used to reproduce a visible image on a recording sheet.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays, or ultraviolet rays, they store part of the energy of the radiation. Then, when the phosphor, which has been exposed to the radiation, is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the amount of energy stored thereon during exposure to the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. Nos. 4,258,264, 4,276,473, 4,315,318 and 4,387,428 and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to radiation, which has passed through an object such as a human body, in order to store a radiation image of the object thereon, and is then scanned with stimulating rays, such as a laser beam, which cause it to emit light in proportion to the amount of energy stored thereon during its exposure to the radiation. The light emitted by the stimulable phosphor sheet upon stimulation thereof is photoelectrically detected and converted into an electric image signal, and the image signal is used to reproduce the radiation image of the object as a visible image on a recording material such as photographic film, on a display device such as a cathode ray tube (CRT), or the like.
In the aforesaid radiation image recording and reproducing systems, in order to eliminate various problems caused by variations in the input information and/or to obtain a radiation image which has good image quality and can serve as an effective tool in, for example, the efficient and accurate diagnosis of an illness, it is desirable to ascertain the characteristics of the image input information before the radiation image is reproduced as a visible image so that the read-out gain can be adjusted to an appropriate value. The characteristics of the image input information depend on the conditions under which an image is recorded, such as the level of the radiation dose used when the image is recorded, on the image input pattern which is determined by what portion of an object (e.g. the chest or the abdomen of a human body) is recorded, and on the image recording method used, such as plain image recording or contrasted image recording. Determining the characteristics of the image input information will hereinafter simply be referred to as "ascertaining the image input information." It is also desirable to adjust the scale factor in accordance with the contrast in the image input pattern in order to optimize the resolution of the reproduced visible image. Also, in cases where image processing, such as gradation processing, is carried out on the read-out image signal, it is desirable to adjust the conditions under which an image is processed in accordance with the characteristics of the image input information.
The method as disclosed in U.S. Pat. No. 4,527,060 may be used to ascertain the image input information before the visible image is reproduced. In the disclosed method, a preliminary read-out operation (hereinafter referred to as the "preliminary readout") is carried out to ascertain the image input information of a radiation image stored on a stimulable phosphor sheet. During the preliminary readout, stimulating rays are used which have an energy level lower than the energy level of the stimulating rays used in a final read-out operation (hereinafter referred to as the "final readout"), which results in a visible image which can be used, particularly for diagnostic purposes. After the preliminary readout is completed, the final readout is carried out.
However, in cases where a preliminary readout is carried out, the processing required to complete the readout takes a long time. Therefore, recently, a novel radiation image processing method has been proposed, which enables a visible radiation image having good image quality to be reproduced, and which, in particular, makes the visible image effective as a tool in the accurate and efficient diagnosis of an illness, without a preliminary readout being carried out. With the proposed radiation image processing method, the emitted light detection range is adjusted so that it is relatively wide during the read-out operation, which results in a visible image which can be used for diagnostic purposes. The optimum value of the read-out gain is calculated from a read-out image signal obtained from the read-out operation, and the read-out image signal is transformed on the basis of the optimum read-out gain into the image signal which would have been obtained if the optimum read-out gain had been used in the read-out operation. By way of example, the optimum read-out gain is set so that the mean value Sk of the read-out image signal range Smin to Smax, which read-out image signal is used to reproduce a visible image, corresponds to the mean value of a predetermined image signal range Qmin to Qmax, which is fed into an image reproducing apparatus. The optimum read-out gain is calculated as a function of the characteristic value (mean value) Sk of the read-out image signal range Smin to Smax. For example, in cases where the probability density function of the read-out image signal is indicated by Curve A in FIG. 3, the mean value of the read-out image signal range Smin to Smax is equal to Sk, and the read-out image signal range Smin to Smax should be transformed into an image signal range Qmin to Qmax with a scale factor Gp (which is the slope of the transformation straight line H in FIG. 3), the image signal components falling in the read-out image signal range Smin to Smax may be transformed so that the value Sk becomes equal to a value Sk', i.e. so that the probability density function of the resulting image signal is indicated by Curve A' in FIG. 3.
The scale factor Gp determines the latitude of a reproduced visible image. In cases where a preliminary readout is carried out, the scale factor Gp has heretofore been adjusted on the basis of image input information ascertained from an image signal obtained from the preliminary readout. During the final readout, the read-out image signal has heretofore been transformed in accordance with the scale factor Gp. However, in cases where transformation processing is carried out to correct the read-out gain as described above, transformation processing can be and often is simultaneously carried out to determine the latitude. Basically, the characteristic value Gp, which is used in the transformation processing carried out to determine the latitude, and the characteristic value Sk, which is used in the transformation processing carried out to correct the read-out gain, can be calculated when the minimum value Smax and the maximum value Smin of the image signal range are known. Therefore, in order to calculate the characteristic values Sk and Gp, the minimum value Smax and the maximum value Smin may be found through, for example, averaging processing performed on the whole read-out image signal obtained from the read-out operation, wherein, for example, the emitted light detection range is adjusted so that it is relatively wide. After the characteristic values Sk and Gp are calculated, they are used to carry out transformation processing expressed by the formula, for example, EQU Q=Gp.multidot.(S-Sk)+Qcenter (1)
where Qcenter denotes the center value of the output image signal range (the center value Qcenter is 511 in cases where the level of the output image signal ranges from 0 to 1023). In this manner, transformation processing for the correction of the read-out gain and transformation processing for the determination of the latitude can be carried out simultaneously.
In general, in order to obtain a reproduced visible image which has better image quality and can serve as a more effective tool, in particular, in the efficient and accurate diagnosis of an illness, transformation processing based on the characteristic values Sk and Gp as well as various other types of processing are carried out; for example, gradation processing, frequency emphasis processing, enlargement/reduction processing, and RI (Radio Isope) removal processing (which is carried out to remove those noise components from a read-out image signal, that are generated by environmental radiation, or the like, and cause black dots to arise in a reproduced visible image) may be carried out.
Heretofore, after the read-out operation is finished and the characteristic values Sk and Gp are calculated, the various types of image processing described above are carried out nearly simultaneously with the reproduction of the visible image (specifically, slightly before the visible image is reproduced).
However, when many types of image processing are carried out during the reproduction of a visible image, a long time is required until the image processing is completed. As a result, the speed with which a visible image can be reproduced becomes low, and reproducing a visible image takes a long time.